The present invention relates to dampers for an engine and a propulsion pump which are mounted on a personal watercraft.
High torsional stress caused by torque acts on a crankshaft of an engine, and in particular of an automobile engine, which needs to rotate at a high speed. In order to resist the torsional stress, the crankshaft is designed to have an allowable torsional stress which is higher than the torsional stress acting on the crankshaft. In order to make the allowable torsional stress of the crankshaft higher, it is necessary to increase the sectional area of the crankshaft. This increases the engine weight. However, such increase in weight is contrary to the effort in recent years to lighten the weight of vehicles in order to reduce the fuel consumption and save resources.
When the torsional stress acting on the crankshaft exceeds the allowable torsional stress of the crankshaft, the durability of the crankshaft decreases, and the vibration and noise from the engine due to the torsional stress increases. In the case of an engine in a four-wheeled vehicle, it is known that for reducing the torsional vibration of the crankshaft a damping mass (weight) is fixed through an elastic member to the pulley which drives a cooling fan or a water pump, and is fixed to the front end of the crankshaft.
In the case of a motorcycle engine, as shown in FIG. 9 of the accompanying drawings, it is known that a damping mass 105 may be fixed to the longitudinal outer side portion of a magneto rotor 114 which is fixed to a front end 101a of the crankshaft for reducing the torsional vibration thereof (Japanese Patent Publication H.8-23304).
For personal watercraft, various measures have been taken to damp the torsional vibration caused by the engine rotation. For example, Japanese Utility Model Publication S.53-25062 discloses elastomeric (i.e. synthetic or natural polymers which are elastic, e.g. rubber buffers) members arranged in a coupling, interconnecting a crankshaft of an engine and a propeller shaft of a propulsion pump, in order to reduce the transmission of the torsional vibration of the engine to the pump.
This coupling, as shown in FIG. 10 of the accompanying drawings, includes a pair of flanges 102,112 each fixed to the adjacent ends of the crankshaft 101 and propeller shaft 111. The flanges include axially extending claws 102b and 112b. The claws 102b of flange 102 and the claws 112b of flange 112 protrude axially toward the each other, and overlap each other when rotating (overlap each other in an axial direction in FIG. 10). The elastomeric member 113 is interposed between adjacent claws 102b and 112b.
In order to further damp the torsional vibration from the engine of a personal watercraft, it might be possible to provide a damper, as in the case with the foregoing motorcycle, by providing additional space at the front out side portion of the crankshaft. For a personal watercraft, however, it is necessary to arrange the engine and accessories (auxiliary parts) very compactly in the small inner space of a body. More specifically, there is a need to arrange most of the engine, the fuel tank, the battery, the muffler and the other parts in the space defined between the hull and the deck. Furthermore, walls of the bulkhead and the pump chamber are arranged with little room or clearance rearward of the engine. Accordingly, under these circumstances, there is no available space in which the damper can be fitted in the manner that the motorcycle damper is fitted.
Furthermore the dampers of personal watercraft are used in an environment of fresh or saltwater except when the damper is fitted in substantially closed casings, like a crankcase. It is necessary for the damper used in such an environment to be directly visible when daily checks are made, and to be easily replaced.
Many of the engines mounted on personal watercraft are also supplied with fuel through a carburetor. The engine vibration, and in particular the torsional vibration in the normal revolution speed range of the engine makes it difficult to optimally set the fuel ratio of the carburetor for the revolution speed range of the engine, and to maintain this ratio.
Technical problems special to personal watercraft also include the torsional vibration at the propeller shaft of the propulsion pump. This torsional vibration causes cavitation on the surface of the pump impeller blades which reduces the propulsive force. It is therefore desirable to damp the torsional vibration of the propeller shaft as much as possible.